In recent years, mobile communication devices such as mobile phones and mobile information terminals have rapidly spread. Duplexers, especially, high-frequency duplexers are used in mobile communication devices to prevent reception and transmission signals from interfering with each other. A duplexer is commonly composed of a transmission filter and a reception filter as a package to downsize the mobile communication devices. A ladder-type filter may be used as a low-loss filter constituting the duplexer. The ladder-type filter is a filter formed by connecting a serial arm and a parallel arm, each having resonators. A surface acoustic wave (SAW) resonator, or a piezoelectric thin film resonator may be used as the resonator. The piezoelectric thin film resonator does not use a fine pattern such as a comb-shaped electrode included in the SAW resonator, and thus is suitable especially for a high frequency device that supports frequencies greater than or equal to 1 GHz.
The transmission filter and a reception filter constituting the duplexer are required to have matched impedance so as to have high impedance in the other's passband. When the passband of the reception filter is close to the passband of the transmission filter, impedance matching is especially important. A phase shifter may be used for impedance matching. Japanese Patent Application Publication No. 2006-129445 discloses the invention that provides a phase shifter for impedance matching and a filter in a single package.
Insertion of the phase shifter may cause loss of signal. To prevent loss of signal by the phase shifter, impedance matching may be performed without using the phase shifter. However, when impedance matching is performed without using the phase shifter, a ripple may occur in pass characteristics of the filter.